add image_alt field to column image template

Column images in the page builder had no alt text support,
causing critical image-alt a11y violations on pages using them.

Author: jamiefolsom

src/components/ColumnContent.astro

@@ -23,7 +23,7 @@ const BlockTypes = {
 
 ---
 { item.__typename.endsWith(BlockTypes.image) && ( 
-  <img src={item.image} class={clsx(
+  <img src={item.image} alt={item.image_alt || ''} class={clsx(
     'w-full object-cover',
     { 'h-full': item.full_height },
     { 'rounded-md': item.rounded }

tina/content/pages.ts

@@ -512,6 +512,10 @@ const staticSectionTemplates: Template<false>[] = [{
           name: 'image',
           label: 'Image',
           type: 'image'
+        }, {
+          name: 'image_alt',
+          label: 'Image Alt Text',
+          type: 'string'
         }, {
           name: 'full_height',
           label: 'Full height?',